A standard hammer-drill chuck such as described in U.S. Pat. No. 4,491,445 of J. Hunger et al has a spindle centered on, rotatable about, and reciprocal through a predetermined stroke along an axis and a chuck body carried on the spindle and formed with a plurality of guides angularly spaced about the axis. Respective jaws displaceable along the guides between inner and outer positions are operated by an adjustment element on the body that is coupled to the jaws for displacing them between their inner and outer positions.
The spindle has an axial front end that engages the axial rear end of a drill bit held in the generally tubular chuck body between the jaws. During hammer drilling this spindle rotates about the axis and also reciprocates, so that it hammers directly on the rear end of the bit. The spindle is formed with a radially outwardly open notch having a chordal base, and the chuck is provided with an internal chordally extending bolt passing through the notch. The notch is axially somewhat longer than the diameter of the bolt, so this bolt rotationally couples the body to the spindle, but permits the body and spindle to move axially relative to each other through an axial play that is equal to the difference between the bolt diameter and the axial notch length.
The problem with such an arrangement is that when hammer drilling in something particularly soft the axial stroke of the device is at a maximum and the bolt strikes against the ends of the notch. This results in damage to the bolt and/or chuck body. This problem is particularly aggravated when a bit is used which has a cylindrical rather than a grooved shaft, as in such arrangements the bit must be solidly clamped between the jaws.